1. Field of the Invention
The present invention relates to processes for the production of a metallic catalyst-carrier and a catalytic component for the decomposition of harmful gases contained in exhaust emissions.
2. Description of the Related Art
Many processes are known for producing a catalyst carrier for nullifying the harmful constituents contained in automobile exhaust emissions. Recent severe regulations for automobile exhaust emissions have led to demands for a catalyst carrier having a light weight, a higher efficiency, and an excellent durability. Ceramics have been used as a catalyst carrier from the viewpoint of the refractory property thereof, but most ceramics have a low durability and a low permeability to exhaust gas. To solve these problems, a metallic catalyst-carrier and a catalytic component using a metallic catalyst-carrier have been proposed. Namely, Japanese Examined Patent Publication (Kokoku) No. 58-1971 and Japanese Unexamined Patent Publication (Kokai) No. 57-71898 disclose processes in which base metals having specific compositions are used, and in the former, alumina is formed on the base metal surface, or in the latter, a whisker is formed on the surface. The former disclosure has a problem in that both the workability and productivity are poor when attempting to maximize the area in contact with exhaust gas, and the latter disclosure also has a problem of a poor heat resistance, i.e., it does not provide the required effect at temperatures higher than 900.degree. C., due to a severe oxidation thereof.
Japanese Examined Patent Publication (Kokoku) No. 51-47157 discloses a process in which a base metal containing iron as a basic constituent is coated with Al and then heat-treated to form a rough surface layer composed mainly of iron and aluminum, to enable an easy carrying of the catalyst The thus formed compound of iron and aluminum is very hard and brittle, and thus many cracks occur therein during service at elevated temperatures, with the result that oxygen enters through these cracks and oxidizes the base metal to form an oxidized layer between the compound and the base metal, whereby the compound is exfoliated
Further various processes are known for producing a metallic catalyst-carrier by coating a base metal mainly composed of iron with Al or Al-alloy, as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 50-66492 and 54-97593. These Publications do not disclose a steel composition, and merely coating iron with Al provides a heat resistance only at temperatures up to 600 or 700.degree. C. at maximum and cannot provide a heat resistance at the automobile exhaust gas temperature of from 900 to 1100.degree. C. Japanese Examined Patent Publication (Kokoku) No. 58-55815 discloses a process in which JIS SUS304, 303, 316, or 430 stainless steel is plated with Al. This process has a problem in that Al-plating is difficult with usual plating techniques and a special treatment is required, and further, if the Al-plating is advantageously performed, this type of stainless steel merely has a heat resistance at temperatures up to 800.degree. C.
Japanese Unexamined Patent Publication (Kokai) No. 62-95142 discloses a process in which a stainless steel foil is plated with Al to form a metallic catalyst carrier. The Al-plating of a foil results in a relatively higher amount of Al, and a subsequent diffusion treatment generates an intermetallic compound and causes embrittlement.
It is known that a steel sheet having a composition of iron and Cr and other additive metals can be used as a metallic catalyst-carrier, such as the 3 to 40% Cr, 1 to 10% Al, 0 to 5% Co, and 0 to 72% Ni disclosed in Japanese Unexamined Patent Publication (Kokai) No. 53-122693 or the 0 to 20% Cr, 0.5 to 12% Al, and 0 to 3% Y disclosed in Japanese Unexamined Patent Publication (Kokai) No. 52-126692. These compositions have a problem in that the addition of Y, Co or other expensive elements is required, or where these expensive elements are not required, an increased Al content for ensuring a high heat resistance causes a high hardness which makes rolling and working, etc., practically impossible.
Japanese Unexamined Patent Publication (Kokai) No. 62-11547 discloses a metallic catalyst-carrier using a foil produced by plating pure Al on a base metal of a ferritic alloy containing Cr and a small amount of Al, and then rolling the Al-plated base metal. The disclosed method for plating Al on such a base metal may leave non-plated portions, and thus the resulting catalyst carrier will suffer from local oxidation.
For a metallic catalyst-carrier, it is most important to provide a reduced flow resistance to emissions, an increased surface area, and a maximum amount carried of a catalyst, and to ensure the catalytic effect, and at the same time, to provide a long term durability under elevated temperatures and vibration, etc., to maintain the catalytic effect over a long period of service. The known compositions for a metallic catalyst-carrier able to maintain a good heat resistance contain high amounts of Cr and Al, and a material having such a composition has a high hardness and is too brittle to be worked into a net, a foil, or other forms for maximizing the catalytic effect, which makes it difficult to use such a material as a catalyst carrier and, in turn, as a catalytic component.